A marine riser is a tubular column used in off-shore drilling operations for oil an gas. It is installed between an underwater well and a floating vessel or semisubmersible facility at the surface of the water. The purpose of the riser is to guide the drillstring to and from the subsea wellhead and to provide means for circulation of drilling fluid. Typically, the marine riser is connected to a subsea wellhead using a wellhead connector. Examples of wellhead connectors are described on pages 1266, 1276, 1277, 1640, and 1642 of the Composite Catalog of Oil Field Equipment and Services, 1984/85 version, Vol. 1. One type of wellhead connector has at the top, a flange for receiving the riser, and at the bottom, locking dogs or securing the wellhead connector to the subsea wellhead. The wellhead connector also includes a sealing element, positioned adjacent the wellhead connector internal shoulder, for preventing the fluid inside the marine riser from leaking out into the surrounding seawater. Once the wellhead connector is operatively secured and sealed to the subsea wellhead, the flow of drilling fluid from the platform through the marine riser and wellhead can begin.
When drilling offshore, unexpected encounter of high pressure gas can result in a well kick that constitutes an emergency. Sometimes the gas kick can be controlled with conventional subsea blowout preventers and pumping kill muds or seawater into the well. Sometimes there is no blowout preventer or the blowout preventer cannot be closed, then the gas kick may be controlled with diverters, as shown in Roche, J. "Subsea Diverters Handle Shallow Gas Kicks." Ocean Industry, (November 1986), pp. 41-44. Oftentimes, the gas kick is so threatening to personnel on the offshore structure that the riser must be disconnected from the subsea wellhead to allow the gas kick to dissipate into the surrounding seawater.
It has been observed that the process of disconnecting a marine wellhead connector from a subsea wellhead becomes increasingly difficult when the wellhead connector and attached riser contains substantially gas rather drilling mud. It has also been observed that riser collapse occurs more frequently when the riser contains substantially gas rather than drilling mud. The disconnection problem has been attributed to mechanical binding of the wellhead connector and the subsea wellhead when the two are not completely aligned. The rise collapse problem has been attribute to the large pressure differential across the riser. See Erb, P.R. "Riser Collapse- A Unique Problem in Deep Water Drilling", IADC/SPE 11394, 1983.
The inventor has determined that the difficulty of disconnect operations experienced when the riser contains substantially gas may also be attributable to a "suction cup" effect occurring at the wellhead connector. Further, tee inventor has determined that the increase in the frequency of riser collapse may be attributable to weakening of the riser resulting from the increase in tension required to separate the wellhead connector from the subsea wellhead i order to overcome the "suction cup" effect.
The fluid forces acting n the wellhead connector have a direct effect on the ease of separating the wellhead connector from the subsea wellhead during disconnect operations. These fluid forces are directly proportional to the pressures acting internally and externally to the wellhead connector. Thee pressures are described in detail below.
As illustrated in FIG. 1, Pi is the internal riser pressure. It varies depending on the type of fluid contained within the riser, and is related to the hydrostatic head of the fluid in the rise. Initially, Pi is large because the riser is full of drilling mud. When a gas kick is encountered Pi drops significantly. However, Pi will begin rising again when the riser and wellhead connector are disconnected and the seawater enters the riser. Ps is the pressure acting on the internal shoulder of the wellhead connector. When the wellhead connector is locked closed, Ps is equal to the external water pressure Po. However, when the wellhead connector is lifted to a position sufficient to establish fluid communication between the internal shoulder of the wellhead connector and the subsea wellhead, Ps becomes equal to Pi. This occurs because the fluid passage between the inside of the riser and the shoulder is not impaired ,while the passage from the shoulder to the seawater is impaired by locking dogs and the clearance between the wellhead and the connector, see FIG. 5. Ps will begin rising again when the riser and wellhead connector are fully disengaged from the subsea wellhead. Po is the external hydrostatic head f the seawater acting at the wellhead connector, and it is constant.
Ps is proportional to a force lifting the riser, thereby aiding in the disconnect operation, whereas Po is proportional to the force that pushes the riser and wellhead connector toward the subsea wellhead, thereby impeding disconnect operations. When Ps is equal to Po, the pressure forces balance each other and the tension force required to raise the riser is a function of only the weight of the riser in water plus any frictional forces. When Ps is smaller than Po, for example when the riser contains substantially gas and the shoulder of the wellhead connector is in fluid communication with the inside of the riser, the tension force required to raise the riser is greater. This is the "suction cup effect" mentioned earlier. This invention described herein is a solution to the problem of how to offset the increase in tension force required to disconnect the wellhead connector and riser from the subsea wellhead when Ps is substantially smaller than Po.
The increase in tension force required to separate a gas-filled riser from a subsea wellhead is similar to the increase in force required to remove a suction cup from a flat surface after the the suction cup has been depressed. For this reason the phenomenon is referred to as the "suction cup" effect. In the example of the suction cup, the pressure acting to push the suction cup toward the surface is only about 14 lb/sq in., the atmospheric pressure. However, in a subsea environment, the pressure acting to push the riser towards the wellhead can run as high as 1300 lb/sq in. in 3000 feet of water.
During riser disconnect operations, the less tension required to lift he riser the better. The objective is to avoid tension stressing the riser to the point where there is a risk of riser collapse. The risk of riser collapse during riser disconnect operations substantially increases when the riser is gas-filled because the riser is subject to an external hydrostatic pressure larger than the internal hydrostatic pressure. Since greater tension must be applied to lift the gas-filled riser, the risk of riser collapse is increased.
To better understand why a riser filled with gas is more difficult to disconnect than a riser filled with liquid, it is helpful to study the forces acting on a riser full of seawater. When the riser is full of seawater, the pressure on the inside of the riser Pi is the same as the pressure Po on the outside of the riser. After he riser is lifted to a position sufficient to establish fluid communication between the wellhead connector and the subsea wellhead thereby creating a gap, the pressure Ps acting on the internal shoulder of the wellhead connector is equal to Pi; consequently, Ps is equal to Po. Therefore, a balance of fluid forces is achieved and the forces acting on the wellhead connector effectively cancel each other. As a result, the tension force required to separate the wellhead connector and riser from the subsea wellhead can be described by the following equation: EQU T.sub.1 =W.sub.br +E (1)
where W.sub.br is the riser buoyant weight, and E is the frictional force.
In the case where the riser is full of gas, the internal riser pressure Pi is equal to the hydrostatic head of the gas and is substantially less than the external hydrostatic pressure Po of the seawater. When the wellhead connector locking dogs are released and while the seal between the wellhead connector and the subsea wellhead is intact, the force required to lift the riser is: ##EQU1## where
Po equals seawater pressure outside the wellhead connector at the level of the sealing element,
Pi equals gas pressure at the level of the sealing element;
D.sub.i equals the inside diameter of the riser; and
D.sub.s equals the outside diameter of the actual seal.
In general, T.sub.2 is not much larger than T.sub.1 since typically D.sub.s is slightly larger than D.sub.i.
After tension T.sub.2 is applied to the riser, the seal between the wellhead connector and the subsea wellhead breaks, and a gap develops between the shoulder of the wellhead connector and the wellhead. Ps becomes approximately equal to the relatively small internal gas pressure Pi, while Po remains the relatively large external hydrostatic pressure of the seawater. This period of unbalanced forces acting on the wellhead connector continues until the gap between the wellhead connector and the subsea wellhead is large enough for a substantial amount of seawater to enter the riser and equalize Po and Ps. This usually occurs after the wellhead connector has been fully disengaged from the subsea wellhead.
In order to achieve the point of disengagement, the tension applied to lift the riser is: ##EQU2## where D.sub.d equals the wellhead connector dogs' inside diameter.
The tension T.sub.3 required to lift the riser when the riser is full of gas and the seal between the wellhead connector and the subsea wellhead is broken, is larger than the tension T.sub.2 required to lift the riser when the riser is full of gas and the seal is intact. The reason is this seal isolates the internal shoulder of the wellhead connector from the inside of the riser; therefore, Ps is approximately equal to Po when the seal is intact, and Ps is approximately equal to Pi when the seal is broken. The tension required to lift the riser shifts from T.sub.2 to T.sub.3 immediately after the riser has been lifted to a level sufficient to provide fluid communication between the shoulder of the wellhead connector and the inside of the riser, but not sufficient to allow for fluid communication between the internal shoulder of the wellhead connector and the external seawater. The fluid flow communication between the external seawater Po and the internal shoulder, where Ps is acting, will continue to be impaired by the narrow clearance between the subsea wellhead and the locking dogs until there is complete disengagement of the wellhead connector from the subsea wellhead. Until this occurs, the "suction cup" effect is experienced during disconnect operations.